1. Field of the Invention
The present invention relates to a check valve which allows a fluid to run only in one direction and, more particularly, to a check valve and a seating valve having good sealing performance.
2. Description of the Related Art
Diesel engines have been widely known as the type of internal-combustion engines called "cylinder injection internal-combustion engines" or "direct injection internal-combustion engines" wherein fuel is injected in engine cylinders. In recent years, the cylinder injection type has been proposed also for spark ignition engines or gasoline engines. There is a tendency in such a cylinder injection internal-combustion engine to increase fuel injection pressure to produce particulate fuel spray to shorten the fuel injection period in order to achieve higher performance of the engine and to reduce emission gas. An engine provided with a supercharger requires a high fuel injection pressure to match charging pressure at the time of supercharging. For this reason, the fuel supply system in the cylinder injection internal-combustion engine is adapted to provide a sufficiently high fuel injection pressure of, for example, about 10 atm.
FIG. 12 is a system diagram showing a conventional fuel supply system. In FIG. 12, a delivery pipe 1 has injectors 1a of the number corresponding to the number of the cylinders of an engine, which is not shown. A high-pressure fuel pump 3 is disposed between the delivery pipe 1 and a fuel tank 2, the delivery pipe 1 and the high-pressure fuel pump 3 being connected by a high-pressure fuel passage 4. The high-pressure fuel pump 3 and the fuel tank 2 are connected by a low-pressure fuel passage 5.
A drain 6 of the high-pressure fuel pump 3 is routed back to the fuel tank 2. A check valve 7 is provided in the high-pressure fuel passage 4 on the discharge end of the high-pressure fuel pump 3. Provided between the high-pressure fuel passage 4 and the low-pressure fuel passage 5 is a bypass passage 8 to bypass the high-pressure fuel pump 3. A check valve 9 is provided in the middle of the bypass passage 8. The high-pressure fuel pump 3, the bypass passage 8, the check valve 7, and the check valve 9 are integrally constituted to make up a high-pressure fuel pump unit 100.
A low-pressure fuel pump 10 is provided at the end on the side of the fuel tank 2 of the low-pressure fuel passage 5, a filter 11 being provided at the fuel inlet of the low-pressure fuel pump 10. A check valve 12 is provided in the low-pressure fuel passage 5 on the discharge end of the low-pressure fuel pump 10. A low-pressure regulator 14, is provided in the low-pressure fuel passage 5 between the high-pressure fuel pump 3 and the low-pressure fuel pump 10, a filter 15 being provided at the fuel inlet of the low-pressure regulator 14. A drain 16 of the low-pressure regulator 14 is routed back to the fuel tank 2.
The delivery pipe 1 has the high-pressure fuel pump 3 and a high-pressure fuel passage 18 at the opposite end; the high-pressure fuel passage 18 is provided with a high-pressure regulator 20. A drain 21 of the high-pressure regulator 20 is routed back to the fuel tank 2.
FIG. 13 is a cross-sectional view illustrative of the details of the conventional check valve 9; and FIG. 14 is a cross-sectional view of a seating valve. In FIG. 13 and FIG. 14, the check valve 9 is provided in the middle of the bypass passage 8 formed in a casing 3a of the high-pressure fuel pump 3. The check valve 9 will be described in detail. The casing 3a has an approximately cylindrical recessed section 25; the low pressure end or the upstream end of the bypass passage 8 is communicated with a bottom surface 25a of the recessed section 25, while the high pressure end or the downstream end of the bypass passage 8 is communicated with a side surface 25b.
A seating valve 30 is provided at the bottom of the recessed section 25. The seating valve 30 is constituted by a main body 30a which is approximately cylindrical and which has a passage bore 30b formed along the central axis thereof, and a neck 30c which is vertically formed over the full outer periphery of the passage bore 30b on one end surface of the main body 30a. The inner periphery of the distal end of the neck 30c is formed into a slope of about 45 degrees, a seat 30d being formed thereon. An O-ring groove 30f is formed in the outer peripheral surface of the main body 30a, an O-ring 34 being placed in the O-ring groove 30f. The seating valve 30 is fixed to the bottom surface 25a of the recessed section 25 by a shoulder 30e of the main body 30a being pushed by a cap 31.
The cap 31 is approximately cylindrical and it has a recessed section 31a on one end thereof, which is formed along the central axis from an end surface; it is disposed in the recessed section 25 such that it holds the recessed section 31a toward the bottom of the recessed section 25. An O-ring groove 31d is formed in the outer peripheral surface of the cap 31, an O-ring 35 being placed in the O-ring groove 31d. The recessed section 25 and the cap 31 together form a holding space 36 provided in the middle of the bypass passage 8. A cylindrical section 31b formed around the recessed section 31a of the cap 31 has a through hole 31c through which fuel passes. The cap 31 pushes the shoulder 30e of the seating valve 30 at the distal end of the cylindrical section 31b. The other end of the cap 31 is fixed by a plate 38, the plate 38 being secured to the casing 3a with screws 39.
The cap 31 has a ball 32 housed in the recessed section 31a, a spring 33 being provided in a compressed state between the ball 32 and the cap 31. The ball 32 has its entire surface sealed; it comes into contact with the seat 30d. The spring 33 normally pushes the ball 32 against the seat 30d; however, when the fuel pressure at the low pressure end or the upstream end of the bypass passage 8 grows higher than the restoring force of the spring 33, the spring 33 is compressed to allow the fuel to pass through.
The check valve 7 has the same constitution described above.
In the fuel supply apparatus having such a constitution, the fuel which has been pressured to a certain degree by the low-pressure fuel pump 10 is further pressured by the high-pressure fuel pump 3 before it reaches the delivery pipe 1 and injected through the injector 1 a into a cylinder of an engine, which is not shown. At this time, the discharge pressure of the low-pressure fuel pump 10 is stabilized to a predetermined range by the low-pressure regulator 14, while the discharge pressure of the high-pressure fuel pump 3 is stabilized to a predetermined range by the high-pressure regulator 20.
In general, the low-pressure fuel pump 10 is driven by a motor or the like and it runs constantly as long as the power is ON, whereas the high-pressure fuel pump 3 is driven by the engine. Hence, when the engine is started, the fuel cannot be supplied smoothly from the low-pressure fuel passage 5 through the high-pressure fuel pump 3 to the high-pressure fuel passage 4. To permit smooth fuel supply, the bypass passage 8 is provided between the high-pressure fuel passage 4 and the low-pressure fuel passage 5 as described above.
While the engine is running, the high-pressure fuel pump 3 is driven and the high-pressure fuel passage 4 and the high-pressure fuel passage 18 are filled with high-pressure fuel. When the engine is stopped, the supply of pressure to the high-pressure fuel passage 4 and the high-pressure fuel passage 18 is cut off. At this time, if the pressure in the piping runs out, then the temperature at which the fuel evaporates lowers, causing the fuel to evaporate. The evaporation of the fuel adversely affect the starting performance when the engine is started next; therefore, the high-pressure fuel passage 4 and the bypass passage 8 are provided with the check valve 7 and the check valve 9 so that the pressure in the high-pressure fuel passage 4 and the high-pressure fuel passage 18 is maintained at a predetermined level even when the high-pressure fuel pump 3 is stopped.
In the check valve 9 with such a constitution, the seating valve 30 is firmly secured to the bottom surface 25a, of the recessed section 25 to survive the high pressure in the bypass passage 8 and to ensure sealing. Since the shoulder 30e of the seating valve 30 is firmly pressed by the cap 31, the seating valve is deformed. The deformation has been posing a problem because it impairs the sealing performance of the seat 30d and the check valve 9. The same applies to the check valve 7.
FIG. 15 is a sectional view illustrating the deformed seating valve 30. The cap 31 pushes a load point I of the shoulder 30e of the seating valve 30 with a force applied in the direction of the arrow shown in the drawing. This cases the seat 30d to deform by a deformation amount 11; specifically, the seat is deformed outward.
Generally, in order to improve the sealing performance, the seating valve 30 and the ball 32 are preliminarily fitted before they are assembled into the check valve 9. More specifically, the ball 32 is pressed against the seat 30d and the contact portion of the seat 30d is slightly dented. When the seat 30d is deformed as mentioned above, the ball 32 seats at the different contact portion from that described above, failing to improve the sealing performance.
The distal ends of the shoulder 30e of the seating valve 30 and the cap 31 inevitably incur slight dimensional errors although they are machined with high accuracy; the force pushing the shoulder 30e of the seating valve 30 does not stay constant in the peripheral direction because of the dimensional errors. As a result, the amount of deformation 11 shown in FIG. 15 is not constant along the periphery; the deformation undulates. This makes it easier for a gap to be produced between the ball 32 and the seat 30d with consequent deteriorated sealing performance.